EP0802633B1 - Circuit logique selon un diagramme de décision binaire - Google Patents

Circuit logique selon un diagramme de décision binaire Download PDF

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Publication number
EP0802633B1
EP0802633B1 EP19960302648 EP96302648A EP0802633B1 EP 0802633 B1 EP0802633 B1 EP 0802633B1 EP 19960302648 EP19960302648 EP 19960302648 EP 96302648 A EP96302648 A EP 96302648A EP 0802633 B1 EP0802633 B1 EP 0802633B1
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node
bdd
branches
clocking
electron
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EP0802633A1 (fr
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Kazuo Nakazato
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Hitachi Europe Ltd
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Hitachi Europe Ltd
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03K—PULSE TECHNIQUE
    • H03K19/00—Logic circuits, i.e. having at least two inputs acting on one output; Inverting circuits
    • H03K19/02—Logic circuits, i.e. having at least two inputs acting on one output; Inverting circuits using specified components

Definitions

  • This invention relates to a binary decision diagram (BDD) logic device.
  • Binary decision diagram (BDD) logic is now commonly used to construct large scale integrated (LSI) circuits.
  • Transistor logic circuits based on BDD architecture have several advantages as compared with the usual AND/OR combinational circuits: higher packing density, lower power consumption, and higher speed. See, for example, K Yano, Y Sasaki, K Rikino and K Seki, IEEE 1994 Custom Integrated Circuits Conference, p. 603.
  • a binary decision diagram represents a digital function as a directed acylic graph with each node labelled by a variable. It provides a complete and concise representation for most digital functions encountered in logic-design applications. For details, reference is directed to S. B. Akers, "Binary decision diagrams,” IEEE Trans. Comput., vol. C-27, no. 6, p. 509, 1978, and R. E. Bryant, "Graph-based algorithm for Boolean function manipulation,” IEEE Trans. Comput., vol. C-35, no. 8, p.677, 1986.
  • a BDD is a graph composed of many nodes Nd i and two leaves L i .
  • each node is represented by a circle Nd containing the variable X 1 , X 2 , X 3 with two branches B labelled 0 and 1, and leaf L 1 and L 2 is represented by a square containing the value "0" or "1".
  • the diagram is entered at the root R and proceeded through downward to a leaf L.
  • Figure 2 shows an example of a simple BDD logic device which makes use of the decision diagram shown in Figure 1.
  • Each node has an input and first and second outputs that are connected to first and second branches respectively.
  • node Nd1 as an example, it has an input Nd1 in and outputs Nd1 out1 , Nd1 out2 that are connected to first and second branches B1, B2 respectively.
  • the nodes Nd2 and Nd3 have a corresponding architecture with the branches being connected as shown.
  • a path through the BDD logic device can be traced from the root R to a leaf L.
  • a signal travels along the path and reaches a "0" or "1" leaf and the value of the function is determined by detecting which leaf the signals reaches.
  • the signal is referred to hereafter as a messenger signal and is injected into the device by an injector at the root R shown in Figure 2.
  • Each of the nodes Nd1, Nd2, Nd3 includes a BDD switching device BDD1-3, which switches the messenger signal between the branches connected to the node output according to the state of the input signal X i .
  • Detector circuits D1, D2 are disposed at the leaves L1, L2 in order to detect the messenger signal.
  • An amplifier in the form of a comparator A provides a final output. It will be understood that many different BDD trees can be constructed using the BDD devices such as BDD1 in a suitable cascade of nodes Ndi coupled between the root R and the leaves L.
  • the switching nodes have been implemented with transistors but with large cascades of BDD devices, the transistors can take up a significant amount of space even on an LSI circuit and can consume considerable levels of power.
  • the present invention provides an improved BDD logic device which overcomes these problems.
  • a BDD logic device including means defining a root, leaves to provide data outputs, branches to convey messenger signals from the root to the leaves, and at least one node for switching signals between first and second ones of the branches selectively in dependence upon applied input signals, the node being adapted to receive electrical charge to a level limited by Coulomb blockade, potential barrier means coupled between the node and the first and second branches respectively, clocking means to provide a clocking waveform that causes charge carrier tunnelling between the node and the first or second branches, and means for altering the relative levels of the potential barrier means selectively in dependence upon the applied control signals whereby to cause charge carriers preferentially to tunnel between the first or the second branch and the node selectively.
  • the present invention permits the nodes in the BDD logic device to be constructed on a nanometre scale so that the messenger signals may comprise individual or small numbers of charge carriers e.g. electrons. This arises because the node is adapted to receive charge at a level limited by Coulomb blockade.
  • the principle of Coulomb blockade will now be described. For a nanofabricated island, if one electron is added to the island, the charging energy will increase. Consequently, an electron cannot enter the island when the charging energy is larger than the thermal energy. This is known as Coulomb blockade. Using this effect, Fulton and Dolan [T. A. Fulton and G. J. Dolan, Phys. Rev. Lett.
  • Geerligs et al. demonstrated the transfer of electrons one by one [ L. J. Geerligs, V. F., Heatgg, P. A. M. Holweg, J. E. Mooij, H. Pothier, D. Esteve, C. Urbina and M. H. Devoret, Phys. Rev. Lett. 64 (1990) 2691], synchronised with an external alternating gate voltage.
  • a single-electron box with one stable state [P. Lafarge, H. Pothier, E. R. Williams, D. Esteve, C. Urbina and M.
  • the potential barrier means between the node and the two branches comprise multiple tunnel barrier configurations. They may be provided with side gates. The input signals may be applied to the side gates.
  • the or each tunnel barrier configuration may include a conductive channel that has been formed from a ⁇ -doped layer in a substrate, the channel including a region of restricted width which exhibits the characteristics of a multiple tunnel junction.
  • the side gate may comprise a conductive track that overlies the conductive channel.
  • FIG. 3 this illustrates a configuration that is used according to the invention in a BDD node for use in a BDD logic tree e.g. as shown in Figure 2.
  • the node configuration includes an input 1 for the messenger signal, a node 2 and output connections 3, 4 for connection to first and second of the branches of the BDD logic tree.
  • Multiple tunnel junctions devices MTJ1, 2, 3 are connected between the terminals 1, 3 and 4, to the node 2, as shown.
  • Each of the multiple tunnel junction devices receives a bias voltage V i from a voltage source 5, that is applied to a side gate 6.
  • V i bias voltage
  • a clocking voltage V c , from a source 7 is applied through a capacitance C g to the node 2.
  • a messenger signal which comprises one or a small group of electrons e.g. ⁇ 10, tunnels through the barrier configuration of MTJ1 to the node 2.
  • electrons which form the messenger signal are caused to tunnel out of the node 2 either to terminal 3 or terminal 4 through the barrier configurations of MTJ2 or 3.
  • the route selected depends on the relative values of the side gate voltages V 2 , V 3 .
  • multiple tunnel junction device means a device with more than two series connected tunnelling junctions.
  • the detailed structure of a MTJ will now be considered with reference to Figures 4 and 5 in order to explain its characteristics.
  • Equation (3) is plotted in Figure 5 and consists of a series of parallel lines for different values of n, shown in dotted outline.
  • a Coulomb Blockade regime -Q c /C ⁇ V ⁇ Q c /C
  • electrons cannot enter or exit the memory node 2 and the upper and lower limits of the voltage ⁇ V c of the memory node 2 that are set by Coulomb Blockade, corresponding to ⁇ Q c /C, are shown by chain lines in Figure 5.
  • V reaches the boundary of this Coulomb Blockade regime, one electron enters or leaves to keep the energy of the node 2 inside the Coulomb Blockage regime.
  • V g By applying a gate-voltage pulse V g with magnitude larger than e ⁇ /C g , the number of electrons on the node 2 can be changed as will be explained in more detail hereinafter.
  • the resulting characteristic exhibits hysteresis and is represented by the solid line shown in Figure 5.
  • one bit of information can be represented by + n and - n electron number states, where n is given by the integer part of ( ⁇ +1)/2. If the capacitances are chosen to satisfy the condition ⁇ 1, a binary code can be represented by the presence or absence of one electron.
  • FIG. 6 the disposition of the Coulomb blockade voltage regions 8 1 , 8 2 and 8 3 for MTJ1, MTJ2 and MTJ3, is shown in respect of the circuit configuration shown in Figure 3. It will be understood that a clocking voltage V c1 is applied to the node 2, which, as shown in Figure 6, varies between a minimum value of V c1 min and a maximum value of V c1 max .
  • the limit is set by the Coulomb blockade limit voltage for MTJ2, namely +V c2 .
  • V c1 clocking voltage
  • the Coulomb blockade voltage +V c2 and the corresponding voltage +V c3 are set by means of the side gate voltages V 2 and V 3 for the MTJs respectively. It can therefore be seen that by suitably adjusting the side gate voltages, the level +V c3 can be made ⁇ V c2 , in which case electrons would tunnel preferentially through the barrier of MTJ3 to the terminal 4, rather than through MTJ2 to terminal 3. Thus, electrons can tunnel either to terminal 2 or 3 depending on the settings of the side gate voltages V 2 and V 3 . Furthermore, it will be understood that the number of electrons that tunnel for each cycle of clocking voltage is determined by the voltage excursion of V c1 . The device thus operates as a turnstile in which a fixed number of electrons transfers to and from the node 2 for each cycle of clocking voltage. In this example, 6 electrons are transferred to and from the node for each cycle.
  • a side-gate structure in ⁇ -doped GaAs material may be used, as shown in Figure 7.
  • An elongate electron channel 9 is formed in a ⁇ -doped layer 10 which is situated 30 nm below the surface of a GaAs substrate 11 and is a few atomic layers in thickness.
  • the layer 9 is typically doped with Si to a concentration of 5 x 10 12 cm -2 and the various layers are grown by molecular beam epitaxy (MBE) or metalorganic chemical vapour deposition (MOCVD) techniques. In both processes, the thickness of the dopant layer can be introduced into only one atomic layer, and the resulting material is said to be ⁇ -doped.
  • MBE molecular beam epitaxy
  • MOCVD metalorganic chemical vapour deposition
  • a fine side-gated constriction 12 with a spaced side gate 13 is defined by electron beam lithography and wet etching to a depth of 120 nm to form the MTJ.
  • the resistance of the junction is controllable since the application of a negative voltage to the side gate 13 changes the electron Fermi energy and repels the electrons from the edge of the electron channel.
  • the MTJ was formed by an etched constriction with a mask width of 500 nm and length of 200 nm.
  • the mask width at the narrowest part i.e. side-gate to channel separation
  • the pattern was defined by electron beam (EB) lithography and wetetching.
  • the EB exposure was performed on a 150 nm thick polymethylmetacrylate (PMMA) coated on the ⁇ -doped GaAs wafer.
  • PMMA polymethylmetacrylate
  • the depth of the trench was controlled to 120 nm by adjusting the etch-time.
  • the total capacitance C of the MTJ is estimated to be 5 aF from the observed Coulomb gap voltage e/ 2C ⁇ 15 mV.
  • This characteristic can be explained by postulating a series of micro-segments split by potentials of donor atoms and forming a series of single-electron transistors. The formation of several islands is supported by another measurement in the highly pinched-off region of operation in that negative resistance due to resonant tunnelling among microsegments has been observed, and by the calculation of conductance modelled using standard single-particle recursive Green's function techniques [R. J. Blaikie, K. Nakazato, R. B. S. Oakeshott, J. R. A. Cleaver and H. Ahmed, Appl.
  • a test chip has been constructed consisting of a BDD logic element as described with reference to Figure 3, using side gated MTJs in a ⁇ -doped layer. Clear switching characteristics have been obtained as shown in Figure 9, where a 6 MHz rf signal was applied as a clocking waveform V c1 to the node 2 shown in Figure 3. Currents I 2 and I 3 through MTJ2 and MTJ3 respectively were measured with a voltage of 10 mV across the logic element. The gate voltage V 3 was fixed at zero. When the gate voltage V 2 was set at zero, electrons flowed equally through MTJ2 and MTJ3. When V 2 was decreased, the Coulomb blockade region of MTJ2 expanded and electron flow was switched from MTJ2 to MTJ3.
  • Figure 10 shows an implementation of the logic function (X 1 + X 2 ) X 3 according to the BDD shown in Figure 1, by means of BDD logic elements that employ MTJs in a manner described with reference to Figure 3.
  • Each input signal X comprises a complimentary pair of voltages ⁇ V as shown in Figure 11.
  • the input signal X 1 is applied to the side gates 6 of MTJ2 and MTJ3.
  • the input signal X 2 is applied to the side gates of MTJ4 and MTJ5 respectively.
  • Clocking waveforms ⁇ are applied to various points in the circuit as shown in Figure 12. The clocking waveform is generated as shown schematically in Figure 13.
  • clocking waveforms ⁇ 1 - ⁇ 4 are applied, which have respective phase differences as shown in Figure 13.
  • clocking waveform ⁇ 1 causes discrete packets for electrons to tunnel sequentially through MTJ1 to the node Nd1 and the input signal X1 causes the packets of electrons either to tunnel through MTJ2 or MTJ3 depending on the value (sign) of X 1 , according to the principles described with previously with reference to Figure 6.
  • MTJ1 does not have a side gate in this example as it is not strictly necessary for operation of the device.
  • MTJ6 does not include a side gate.
  • MTJ6 is included to maintain the correct phase relationship between the signals passing through the BDD tree, and MTJ6 cooperates with MTJ3 to operate as a turnstile. Similarly, MTJ7 cooperates with MTJ4 to operate as a turnstile and maintain the correct phase relationship for signals passing through the BDD logic tree.
  • Figure 14 illustrates a modification in which a two-phase clocking regime is used, which permits MTJ6 and 7 to be omitted. This has the advantage of . simplifying the device structure but has the disadvantage of a higher power consumption because the capacitance of each node is higher than in Figure 14 as compared with Figure 10. In practice, a combination of the two-phase and four-phase techniques described with reference to Figures 10 and 14 can be combined in a single circuit.
  • CMOS inverter A1, A2 are responsive to the electron state of the nodes which constitute the leaves L1, L2 so as to provide a binary conventional voltage output at terminals T1, T2.
  • An alternative configuration of the CMOS inverter output stages A1, A2 is shown in Figure 16.
  • Other output stages can be used for example single-electron transistor or a gated MTJ, instead of the CMOS transistor arrangement.
  • an alternative MTJ structure which includes a constriction 15 in a ⁇ -doped layer 10 on a substrate 11, in a conductive track 9 formed according to the principles described with reference to Figure 7.
  • the constriction 15 is formed by bilateral cutout regions 16a, 16b on opposite sides of the conductor 9.
  • a metallic gate layer 17 so that a voltage applied to the gate produces a functionally similar effect to a voltage applied to the side gate 13 shown in Figure 7.
  • the ⁇ -doped layer 10 was grown on the GaAs substrate 11 by MOCVD.
  • the ⁇ -doped layer 10 was formed 30 nm below the surface and was doped with Si to a concentration of 5 x 10 12 cm -2 .
  • the carrier concentration is estimated to be 4 x 10 12 cm -2 from Hall resistance measurements made at liquid helium temperatures.
  • aluminium was evaporated onto the substrate to a thickness of 40 nm, and patterned by a lift-off method, to achieve the configuration shown in Figure 18a.
  • Gold was evaporated to a thickness of 250 nm and patterned by a lift-off method so as to form the gate electrode 17 as shown in Figure 18c. Then, using the deposited A1 layer 18 and the Au layer 17 as a mask, the GaAs substrate was etched to a depth of 120 nm by a reactive ion etch method in SiCl 4 and Ar gases as shown in Figure 18d, removing GaAs from regions 20a, 20b.
  • Metallic single electron transistors can also be used in BDD devices according to the invention. They can be made in a variety of ways. The classical approach was to use the Al-Al 2 O 3 -Al structure with the oxide providing the thin tunnel barrier. The problem of achieving a small size as well as accurate alignment was overcome by using a procedure which gives self-aligned structures.
  • a suspended or free-standing bridge of a material is fabricated by electron beam lithographic techniques.
  • An Aluminium film is evaporated at an angle to the bridge and oxidised to form a very thin insulating layer on its surface.
  • a second film evaporated at an angle overlaps the first film by.a few hundred nanometers if the angle is carefully chosen.
  • Another effective method for making metallic tunnel junctions is to make very thin metallic wires with a discontinuity created by a step coverage break induced by a sharp-edged groove in the substrate.
  • the sharp groove can be created in a SiO 2 layer on Si substrate. Clear Coulomb blockade has been seen in such devices [S. Altmeyer, B. Spangenberg, and H. Kurz, Appl. Phys. Lett. 67 (1995) 569, and W. Langheinrich and H. Ahmed, Proceedings of the 1995 International Microprocess Conference].
  • Single-electron devices can be realised in semiconductor structures by the split-gate method [T. J. Thornton, M. Pepper, H. Ahmed, D. Andrews, and G. J. Davies, Phys. Rev. Lett. 56 (1986) 1198], employing electrostatic squeezing to vary the shape of a two-dimensional electron gas at a GaAs and AlGaAs heterointerface.
  • Meirav et al. fabricated narrow channels interrupted by two controlled potential barriers and having a tuneable electron density [U. Meirav, M. A. Kastner and S. J. Wind, Phys. Rev. Lett. 65 (1990) 771].
  • SOI not only lends itself readily to a variety of fabrication processes which are compatible with future CMOS technologies but allows multiple tunnel junction (MTJ) devices to be made in silicon.
  • MTJ tunnel junction
  • Devices were fabricated on a 50 nm thick silicon layer of a silicon-on-insulator substrate formed by separation by implantation of oxygen (SIMOX).
  • the silicon was uniformly doped to above the metal-insulator transition by implanting 1 x 10 14 cm -2 of phosphorus at 20 keV and was annealed at 950°C for 30 minutes, initially in an oxidising environment to grow at 25 nm thick oxide cap and minimise loss of dopant. The oxide cap was then removed in buffered HF. Electron beam lithography and dry etching were used to define a very narrow wire in the SOI layer, and tunnel junction regions were defined with a second stage of electron beam lithography, and etching perpendicular to the wire length, without etching all the way through the silicon.
  • all of these known devices can be used to isolate small numbers of electrons, typically less than ten, at a node, and to switch the electrons. to different branches in a BDD logic device according to the invention.

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  • Engineering & Computer Science (AREA)
  • Computer Hardware Design (AREA)
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  • General Engineering & Computer Science (AREA)
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  • Insulated Gate Type Field-Effect Transistor (AREA)
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Claims (13)

  1. Dispositif logique de diagramme de décision binaire (BDD) comprenant un moyen définissant une racine (R), des feuilles (L1, L2) pour fournir des sorties de données, des branches (B) pour acheminer des signaux messagers de la racine aux feuilles, et au moins un noeud (Ndi, 2) pour commuter des signaux entre une première et une seconde (B1, B2) des branches de manière sélective en fonction des signaux d'entrée appliqués (Xi), le noeud étant adapté pour recevoir la charge électrique à un niveau limité par le blocage de Coulomb, un moyen de barrière de potentiel (MTJ, 2, 3) couplé entre le noeud et les première et seconde branches respectivement, un moyen de cadencement (Vc1) pour fournir une forme d'onde de cadencement (ϕ1) qui provoque le passage par effet tunnel du porteur de charge entre le noeud (2) et les première et seconde branches (B1, B2), et un moyen (V2, V3) pour modifier les niveaux relatifs du moyen de barrière de potentiel de manière sélective en fonction des signaux de commande appliqués pour faire que les porteurs de charge circulent préférentiellement par effet tunnel entre la première ou la seconde branche et le noeud de manière sélective.
  2. Dispositif selon la revendication 1, comprenant une entrée de signal messager (1) pour le noeud (2) et un moyen de barrière de potentiel (MTJ1) entre l'entrée et le noeud.
  3. Dispositif selon la revendication 2, comprenant un moyen (Vcl, Cg) pour appliquer la forme d'onde de cadencement (ϕ1) au noeud (2).
  4. Dispositif selon l'une quelconque des revendications précédentes, comprenant une pluralité d'éléments logiques BDD (BDD1) couplés les uns aux autres, chacun desdits éléments comprenant un jeu respectif de deux desdites branches, undit noeud associé et un moyen de barrière de potentiel entre les deux branches et le noeud.
  5. Dispositif selon la revendication 4, comprenant un moyen pour appliquer une pluralité de formes d'onde de cadencement (ϕ1, ϕ2, ϕ3, ϕ4) aux éléments logiques BDD avec une différence de phase pour faire que les porteurs de charge se déplacent de noeud à noeud des éléments logiques.
  6. Dispositif selon l'une quelconque des revendications précédentes, dans lequel le moyen de barrière de potentiel comprend de multiples configurations de barrière à effet tunnel (MTJ1, MTJ2, MTJ3).
  7. Dispositif selon la revendication 6, dans lequel les multiples configurations de barrière à effet tunnel sont munies de grilles latérales (6).
  8. Dispositif selon la revendication 7, comprenant un moyen pour appliquer lesdits signaux d'entrée (X1, X2, X3) aux grilles latérales (61, 62, 63).
  9. Dispositif selon l'une quelconque des revendications 6 à 8, dans lequel le moyen fournissant la configuration de barrière à effet tunnel comprend un canal conducteur (10) qui a été formé à partir d'une couche dopée δ dans un substrat (11), le canal comprenant une zone (12, 15) de largeur rétrécie qui offre les caractéristiques d'une jonction tunnel multiple.
  10. Dispositif selon la revendication 9, comprenant un moyen de grille latérale (13, 17) adjacent à la zone rétrécie.
  11. Dispositif selon la revendication 10, dans lequel le moyen de grille latérale comprend une piste conductrice (17) qui recouvre le canal conducteur.
  12. Dispositif selon l'une quelconque des revendications précédentes, comprenant un moyen de sortie (A1, A2) couplé aux feuilles pour fournir un signal logique de sortie.
  13. Dispositif selon la revendication 12, dans lequel le moyen de sortie comprend un amplificateur.
EP19960302648 1996-04-16 1996-04-16 Circuit logique selon un diagramme de décision binaire Expired - Lifetime EP0802633B1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
DE69629275T DE69629275T2 (de) 1996-04-16 1996-04-16 Logische Vorrichtung nach einem binären Entscheidungsdiagramm
EP19960302648 EP0802633B1 (fr) 1996-04-16 1996-04-16 Circuit logique selon un diagramme de décision binaire
JP9571297A JPH1041497A (ja) 1996-04-16 1997-04-14 ２進決定ダイアグラム（ｂｄｄ）論理素子

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Application Number Priority Date Filing Date Title
EP19960302648 EP0802633B1 (fr) 1996-04-16 1996-04-16 Circuit logique selon un diagramme de décision binaire

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EP0802633A1 EP0802633A1 (fr) 1997-10-22
EP0802633B1 true EP0802633B1 (fr) 2003-07-30

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DE19738115C1 (de) 1997-09-01 1999-03-18 Siemens Ag Schaltungsanordnung mit Einzelelektron-Bauelementen, Verfahren zu deren Betrieb und Anwendung des Verfahrens zur Addition von Binärzahlen
GB9724642D0 (en) 1997-11-21 1998-01-21 British Tech Group Single electron devices

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DE69629275D1 (de) 2003-09-04
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EP0802633A1 (fr) 1997-10-22

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